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12.23: Nitrogen Cycle

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    What can bean plants do that most other plants can't?

    No, they don't grow giant stalks to the clouds. Bean plants and other legumes (plants that have their seeds in pods) can use the nitrogen in the air to grow. It takes the help of special bacteria friends in the soil, and this relationship is unique to the legumes.

    The Nitrogen Cycle

    Like water and carbon, nitrogen is also repeatedly recycled through the biosphere. This process is called the nitrogen cycle. Nitrogen is one of the most common elements in living organisms. It is important for creating both proteins and nucleic acids, like DNA. The air that we breathe is mostly nitrogen gas (N2), but, unfortunately, animals and plants cannot use the nitrogen when it is a gas. In fact, plants often die from a lack of nitrogen even through they are surrounded by plenty of nitrogen gas. Nitrogen gas (N2) has two nitrogen atoms connected by a very strong triple bond. Most plants and animals cannot use the nitrogen in nitrogen gas because they cannot break that triple bond.

    In order for plants to make use of nitrogen, it must be transformed into molecules they can use. This can be accomplished several different ways (Figure below).

    • Lightning: When lightning strikes, nitrogen gas is transformed into nitrate (NO3-) that plants can use.
    • Nitrogen fixation: Special nitrogen-fixing bacteria can also transform nitrogen gas into useful forms. These bacteria live in the roots of plants in the pea family. They turn the nitrogen gas into ammonium (NH4+) (a process called ammonification). In water environments, bacteria in the water can also fix nitrogen gas into ammonium. Ammonium can be used by aquatic plants as a source of nitrogen.
    • Nitrogen also is released to the environment by decaying organisms or decaying wastes. These wastes release nitrogen in the form of ammonium.

    Ammonium in the soil can be turned into nitrate by a two-step process completed by two different types of bacteria. In the form of nitrate, nitrogen can be used by plants through the process of assimilation. It is then passed along to animals when they eat the plants.

    Sending Nitrogen back to the Atmosphere

    Turning nitrate back into nitrogen gas, the process of denitrification, happens through the work of denitrifying bacteria. These bacteria often live in swamps and lakes. They take in the nitrate and release it back to the atmosphere as nitrogen gas.

    Just like the carbon cycle, human activities impact the nitrogen cycle. These human activities include the burning of fossil fuels, which release nitrogen oxide gasses into the atmosphere. Releasing nitrogen oxide back into the atmosphere leads to problems like acid rain.

    The nitrogen cycle tracks the flow of nitrogen through an ecosystem
    Figure \(\PageIndex{1}\): The nitrogen cycle includes assimilation, when plants absorb nitrogen; nitrogen-fixing bacteria that make the nitrogen available to plants in the form of nitrates; decomposers that transform nitrogen in dead organisms into ammonium; nitrifying bacteria that turn ammonium into nitrates; and denitrifying bacteria that turn nitrates into gaseous nitrogen.


    • Gaseous nitrogen is converted into forms that can be used by plants during the process of nitrogen fixation.
    • Denitrifying bacteria turn nitrate back into gaseous nitrogen.


    1. How do living organisms use nitrogen?
    2. What is nitrogen fixation? Describe how it happens.
    3. How is nitrate in the soil converted back to nitrogen gas?
    4. How does acid rain form?

    12.23: Nitrogen Cycle is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.

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